SPP1 and breast carcinoma: Raja et al. showed that silencing OPN or its receptor significantly downregulated hypoxia-induced breast cancer cell migration and invasion and that HIF-1α was involved in this process (Le et al., 2003; Vergis et al., 2008; Raja et al., 2014; Ostheimer et al., 2017; Cao et al., 2019; Nishio et al., 2021).